Electromechanical filter

ABSTRACT

Electromechanical filter consisting of several mechanically coupled, mechanical, flexural resonators which are arranged in a housing. The filter is characterized in that a sound absorbing intermediate layer is inserted between the resonators and at least one wall of the housing.

United States Patent 1 Traub July 10, 1973 ELECTROMECHANICAL FILTER [75] Inventor: Karl Traub, Olching, Germany [7 3] Assignee: Siemens Aktiengesellschaft, Munich,

Erlangen, Berlin, Germany 22 Filed: Sept. 21, 1971 21 App1.No.:182,49l-

[30] Foreign Application Priority Data Sept. 30, 1970 Gennany P 20 48 162.3

52 us. (:1. 333/71, 333/72 511 Int. Cl 110311 7/10, H03h 9/00 58 Field of Search 333/70, 71, 72; 84/l.l6

[56] References Cited UNITED STATES PATENTS 3/1950 Adler 333/71 3,049,958 8/1962 Benioff 84/].16 2,799,832 7/1957 Niederman et. a1. 333/71 3,564,459 2/1971 Hahn 333/71 3,028,564 4/1962 Tanaka et al. 333/71 X 3,011,136 11/1961 Scarrott 333/71 X Primary ExaminerRudolph V, Rolinec Assistant Examiner--Saxfield Chatmon, Jr. Attorney- Arthur E. Wilfond, Herbert L. Lerner et al.

[5 7] ABSTRACT Electromechanical filter consisting of several mechanically coupled, mechanical, flexural resonators which are arranged in a housing. The filter is characterized in that a sound absorbing intermediate layer is inserted between the resonators and at least one wall of the housing. 1

1 Claim, 1 Drawing Figure ELECTROMECHANICAL FILTER The present invention relates to an electromechanical filter consisting of several mechanically coupled, mechanical, flexural resonators, arranged in a housing.

As is well known, mechanical filters consist of several mechanically coupled mechanical resonators, in which the filter characteristic is determined by the vibration process of the resonators and couplers. Among the numerous forms of vibration possible for the resonators and the coupling elements, the use of the flexural mode of vibration for the resonator and the use of the longitudinal mode of vibration for the coupling elements is particularly advantageous as in the flexural mode of vibration the resonant frequency of the resonator is codetermined by the area moment of inertia effective in the direction of the vibration and, as with use of longitudinal couplers, relatively large band widths can be obtained even with relatively thin coupling wires. The filter is generally accommodated within a housing for protection against external influences. The air surrounding a mechanical filter is also excited to vibration by the motion of the individual resonators. Because of the low vibration of air, damping of the resonators, therefore, occurs. This damping is particularly pro-' nounced, and sometimes also accompanied by a detuning in frequency if, as a result of the internal dimensions of the housing, air spaces are generated which are in resonance with the operating frequency of the filter. The result is then distortion of the transfer characteristic of the filter at the resonant frequencies of these air spaces.

Through U.S. Pat. No. 2,905,909 it has already become known in a mechanical filter with plate or slabshaped resonators that perform longitudinal or trans versal vibrations to cover the resonators by an inlay of elastic material such as, for instance, rubber or foamlike plastic materials. This covering, however, serves on one hand for mounting the filter body and, on the other hand, to suppress undesired flexural vibration modes of the resonators. A certain pressure of the inlay on the resonators is, therefore, necessary.

It is an object of the present invention to suppress, in a mechanical filter consisting of mechanical flexural resonators, disturbing air resonances within the housing without simultaneously adversely influencing the flexural resonators with respect to their vibration behavior.

This problem is solved, according to the invention, starting from an electromechanical filter consisting of several mechanically coupled, mechanical, flexural resonators,'which are arranged in a housing, by the provision of a sound absorbing intermediate layer inserted between the resonators and at least one wall of the housing.

Flexural resonators, for equal resonator dimensions, in comparison to resonators which execute other forms of vibration in general, have relatively large surfaces which, in the case of flexural vibration, are exposed to the air resistance. For this reason, the disturbing influences of air resonances of the transfer characteristics of the filter occur especially in filters with flexural resonators, if the dimensions of the housing are unfavorable. In the event of phase-opposed vibration of two adjacnt resonators, the air space between them is alternatingly increased and decreased in this filter. Air waves are, thereby excited perpendicularly to the direction of vibration of the flexural resonators. These waves are reflected at the wall of the housing and can form standing waves. These air resonances have a disturbing reaction on the filter characteristics inasmuch as in part substantial attenuation distortion occurs. Although the possibility exists, by suitable choice of the internal housing dimensions, to change the length of the oscillating air columns in such a manner that no air resonances and thereby also no attenuation distortion can occur, sufficiently good results can be obtained only over a relatively small temperature range because of the pronounced temperature dependence of the sound velocity in air. The maximally permissible housing dimensions, as well as their tolerances, further limit the application of this method.

The invention will be described in greater detail with an illustration embodiment making reference to the Drawing, in which:

The single FIGURE shows a mechanical filter consisting of several mechanical flexural resonators 1, which perform longitudinal vibrations, and which are coupled by a continuous mutual coupling element. The end resonators are provided with electromechanical transducers 2 and 2' for conversion of electrical to me chanical vibrations or the conversion of mechanical to electrical vibrations, respectively. The resonators, which originally had circular cross section, are provided with a flat, at which support elements 4 are provided in the region of the vibration nodes. These support elements, in turn, are anchored on a base plate 5. The resonators are shown so that their end face is visible. The coupling element 3 is fastened to the individual resonators in their central region. The resonators, themselves, are arranged in one plane and are excited to perform flexural vibrations in such a manner that they execute these vibrations in the direction of the double arrow 6, Le. parallel to the base plate 5. Consequently, the coupling element 3 also vibrates longitudinally in the direction of the double arrow 6. The filter is surrounded, on all sides, by a housing 7 in order to exclude external influences as far as possible. An intermediate layer 8 consisting of sound absorbing material, which inhibits the air resonances occurring perpendicularly to the direction of vibration 6, is between the filter body and the wall of the housing. In the instant example, plastic material, namely polyester fabric, which has a thickness of 0.5 mm, is used as the material for the intermediatel layer 8. No attenuation distortion, caused by air resonances, was observed with this arrangement in the entire temperature range of interest, that is between 0 to 55C.

In this example, the intermediate layer 8 is placed directly on the resonators and specifically on the side opposite coupling element 3. The intermediate layer 8, therefore, lies parallel to the direction of vibration 6 of the resonators and can extend, if necessary, over the entire length of the resonator as is shown.

Experiments with a substantially thinner fabric of about 0.1 mm, or experiments with a heavily roughened interior surface of the plastic housing show no sufficient improvement. This indicates that the effect of the 0.5 mm thick fabric insert is based mainly on the absorption of sound waves, but less on diffused reflection from the rough surface of this felt-like insert.

It is furthermore particularly advantageous for manufacturing purposes that the intermediate layer 8 need not be rigidly connected with the housing, but can rest, without detrimental effect on the transfer function,

chanically coupled, mechanical, flexural resonators which are arranged in a housing, wherein a sound absorbing intermediate layer is inserted between the resonators and at least one wall of the housing, the coupling of the resonators being achieved via a continuous coupling element which carries out longitudinal vibrations, the intermediate layer being'arranged on the side of the resonators opposite to that of the coupling element. 

1. Electromechanical filter comprising several mechanically coupled, mechanical, flexural resonators which are arranged in a housing, wherein a sound absorbing intermediate layer is inserted between the resonators and at least one wall of the housing, the coupling of the resonators being achieved via a continuous coupling element which carries out longitudinal vibrations, the intermediate layer being arranged on the side of the resonators opposite to that of the coupling element. 